The present invention relates generally to artificial cardiac pacing, and more particularly to improvements in the control of ventricular pacing rates of a dual-chamber pacemaker for various conditions of patient rest, exercise/activity, and atrial dysrhythmia, and with the advantage of automatic mode switching between dual-chamber and single-chamber modes.
In general, dual-chamber pacemakers are designed to maintain atrioventricular (AV) synchrony when implanted in a patient. Two electrodes are used, one for sensing atrial signals and the other for stimulating an appropriate ventricular response. A slight delay is imposed between the ventricular response and the atrial signal that prompted it, to correspond to the normal AV delay of about 150 milliseconds (ms).
With the introduction in the late 1970s and early 1980s of fully automatic DDD pacemakers (three-position ICHD code, and related five-position code adopted by NASPE and other pacing organizations, DDD being indicative of sensing and pacing in both chambers and dual chamber response to a sensed beat), several clinical aims were achievable. Among these aims, the devices permitted adapting the pacing rate of the ventricle to depend on the rate of the sensed intrinsic atrial signal, and the AV synchrony. However, as many as 50% of the patients who had DDD pacemakers implanted experienced problems with atrial sensing or with atrial instability in the presence of underlying atrial rhythm disorders. For that patient population, it was necessary to switch from a DDD pacing mode back to a simple VVI pacing mode (a ventricular demand mode, in which ventricular pacing is inhibited by spontaneous electrical activation of the ventricle).
Despite the development of significant technological improvements over the early dual-chamber pacemakers, in such things as sense amplifiers, electrodes for atrial leads, and timing cycles including dynamic AV delay and refractory periods, a considerable percentage of patients with dual-chamber pacemakers still suffer from inadequate atrial rates. Underlying disease is the primary culprit, with estimates that about one-third of all patients with sick sinus syndrome (characterized by sinoatrial (SA) arrest or SA exit block) have inappropriately high atrial rates accompanying atrial fibrillation, atrial flutter, or sinus tachycardias including atrial reentry tachycardias and ectopic tachycardiac events (developing from a focus other than the SA node), as well as the slow heart rates that call for bradycardia pacing.
In the past, the most widely used proposal to eliminate problems of inappropriate atrial rate in patients with dual-chamber pacemakers was to establish rate criteria to discriminate between physiologic atrial rates and pathologic atrial rates. For example, a high atrial rate evidenced by the atrial beats exceeding a prescribed number over a specified time interval, could be used for switching the pacing mode from, say, an atrial-sensing, ventricular-tracking DDD or VDD mode to a pure ventricular stimulation mode such as VVI or VVI-R (i.e., VVI with rate responsiveness). In the VVI mode, the sensing and pacing functions are active in the ventricle only, so that in the absence of sensed depolarizations within a preset period, the pacemaker generator generates stimulating pulses at a preprogrammed nominal rate (programmed pacing rate); and if spontaneous depolarizations are sensed at a rate faster than the programmed pacing, the stimulation pulse is inhibited.
But mode switching based on such criteria has produced its own set of problems. If the atrial detection rate is set too low, even physiologically-increased atrial rates following physical exercise can cause the system to revert to an inappropriately low rate. If the criterion for detection of inappropriately high atrial rate is set too high, atrial rates occurring with even slow atrial flutter, sinus tachycardia, or ectopic beats may be below the threshold and cause inappropriately high ventricular pacing rates although the patient may be at rest.
It is essential that the dual-chamber device should respond to inappropriate high atrial rate in a manner that precludes pacing the ventricle at an equally fast rate. The maximum pacing rate (MPR) or upper rate limit of the pacemaker is usually programmed to produce an abrupt 2:1 block, which has its own problems. The total atrial refractory period (TARP, which is the sum of the AV interval and the post-ventricular atrial refractory period or PVARP) affects the upper rate limit in that an atrial rate is reached at which no further sensing of atrial beats can occur because they fall in the atrial refractory period. An abrupt 2:1 block, which can occur even in situations where the high atrial rate may be appropriate because of patient exercise and metabolic need, causes an undesirable abrupt change in the patient's cardiac output and blood pressure.
Wenckebach behavior, a progressive lengthening of the P-R interval such that eventually a ventricular beat does not follow an atrial beat, may be developed at the upper rate limit as a means for slowing the paced rate. Some dual-chamber pacemakers are programmable to allow a more gradual reduction in the patient's heart rate when the atrial rate reaches the 2:1 block imposed by the programmed upper rate limit.
A second approach to solving the problem of intermittent high atrial rate is based on sensing the activity or state of exercise of the patient. A rate-adaptive or rate-responsive pacemaker controls the pacing rate based on the output signal of a sensor such as an accelerometer, a temperature-detecting element, a QT-interval detector, or a detector of impedance-derived minute ventilation.
In addition to use for controlling heart rate, activity sensor-based techniques can be used to monitor the pacemaker's stimulation rate or the patient's intrinsic heart rate. For example, the RELAY.TM. dual-chamber, multiprogrammable, rate-responsive cardiac pulse generator manufactured by Intermedics, Inc. (Angleton, Tex.) controls the pacing rate and also monitors the adequacy of atrial-triggered stimulation in the atrium.
Where intermittent rhythm disorders such as atrial fibrillation and atrial flutter cause an inappropriately high atrial rate in the normal, unaided heart, the inability for rapid transfer of the electrical signal through the AV node from the SA node causes the ventricle to be stimulated only every other or even every third atrial beat. This self-correction does not occur with the DDD pacemaker because the device itself senses the high atrial rate and accordingly can pace the ventricle at an inappropriately high rate.
However, the MPR of the pacemaker is programmed in a range (typically, from 150 to 170 bpm) to prevent a dangerously high ventricular response. Still, pacing at the MPR is unacceptable at any time that the patient is inactive. Because of high myocardial oxygen consumption, particularly in the presence of existing coronary stenosis, the patient tends to develop symptoms of cardiac insufficiency, lung congestion, shortness of breath, and angina pectoris.
In some prior art dual-chamber, rate-responsive pacemaker generators, the MPR is supplemented by a lesser interim rate which is greater than the lower rate limit of the device--a ventricular tracking limit (VTL) to which the pacing rate moves from its base rate conditioned on a high atrial sensed rate without patient exercise. This conditional VTL (CVTL) was originally incorporated in a temperature sensing, rate-responsive pacemaker developed in the mid-1980s. A simple algorithm that discriminated between patient rest and exercise based on venous blood temperature was used to control a change in rate to a fixed rate that exceeded the lower rate limit of the device by about 30 bpm.
The RELAY.TM. pacing system uses an accelerometer as a body motion/ activity sensor, and has a CVTL feature which, when programmed "on" (i.e., as an operating condition of the device), undergoes a controlled jump to the CVTL rate when a high atrial rate is sensed without confirmation of patient exercise from the accelerometer. This is considerably less traumatic to the patient than an abrupt move to the MPR. The CVTL is overridden, so long as the MPR is programmed "on", whenever the rate calculated in response to patient exercise exceeds the programmed base pacing rate by a preselected amount--20 bpm, for example. At this exercise (activity) sensor-calculated rate threshold, the pacemaker generator restores the 1:1 AV synchrony up to the MPR. When the sensed atrial rate drops below the activity-calculated rate, the generator performs rate-responsive AV sequential pacing.
The advantage of a CVTL is somewhat moderated by the fact that if an episode of atrial tachyarrhythmia is experienced by the patient during moderate exercise, such as even slow walking, an abrupt rate change can take place from the base rate or current activity-based rate to the MPR of the device. The opposite situation can result when a patient ceases activity--an abrupt drop in rate may occur from the MPR to the CVTL rate. Clearly, this would be undesirable.
Despite the advance in pacing comfort attributable to the CVTL, it would be desirable to further compensate a designated upper rate limit according to changes in the metabolic need of the patient. It is a principal aim of the present invention to provide a rate-responsive, dual chamber pacemaker in which the VTL is implemented to be a dynamic rather than a fixed rate, to minister further to the patient's physiology.
A further object of the invention is to provide a method and apparatus to control the rate at which the ventricle is paced by a dual-chamber pacemaker through several different rate zones, based upon a combination of (i) a dynamic adjustment of the ventricular tracking limit according to an activity signal, and (ii) an automatic mode switch that causes the pacemaker generator to go from a dual-chamber to a single-chamber mode, and to revert to the dual-chamber mode based on an atrial cut-off rate and a programmable rate criterion.